This invention relates to a deactivating device for deactivating shoplifting detection labels of an electronic shoplifting detection system, which labels comprise a resonant circuit with a coil and a capacitor, said deactivating device comprising an antenna circuit comprising an antenna coil tuned with at least one capacitor to the resonant frequency of the resonant circuit, by means of which sufficient energy can be induced in a resonant circuit of a label to effect electrical breakdown in the capacitor thereof.
An electronic shoplifting detection system consists of a plurality of components, viz.:
1. labels, which are attached to the articles to be protected;
2. detection pillars, which are arranged at the exit of a shop and serve to detect the passing labels;
3. a packaging lable detector, which serves to detect labels to be removed when the articles are purchased.
Besides labels which are removed when the articles are purchased, there are labels such as the so called adhesive labels, which are not removed, but must be deactivated, i.e. rendered inactive as a detection label. Such an adhesive label consists of insulating substrate with a track pattern of conducting material provided thereon. This track pattern forms a coil and a capacitor, together forming a resonant circuit. The resonance effect is used to detect the presence of the label. An adhesive label can be deactivated by preventing the resonance. In practice, to that end an electrical breakdown is effected in the dielectric between the capacitor plates, whereby, as a result of electric energy stored in the capacitor, a strong heating occurs very locally, so that a hole is formed in the dielectric material between the capacitor plates, and some conductor material evaporates which precipitates again on the edges of the hole in the dielectric. Thus, a conductive connection is formed between the two capacitor plates, whereby the capacitor is effectively short circuited and the resonance effect disappears. In order to reduce the energy necessary for deactivation, in some manner or other a weak spot is provided in the capacitor during manufacture of the adhesive labels, so that the voltage across the capacitor necessary for breakdown is of the order of 20 V, for instance.
A so-called deactivator is the device which must supply the energy for deactivation of an adhesive label. It is useful to combine a deactivator with a packaging table detector because after the deactivation operation it must be verified that the label has really been deactivated. This function is already provided for by existing packaging table detectors. U.S. Pat. No. 4,498,076 discloses such a deactivator. Further, an activator is disclosed in applicant's Dutch patent application NL 9000186 corresponding to U.S. patent application Ser. No. 07/645,886, filed Jan. 25, 1991, now U.S. Pat. No. 5,153,562. After the resonant frequency of the label to be deactivated has been measured, this high-frequency deactivator momentarily generates a strong high-frequency carrier wave having a frequency which is equal to that resonant frequency. This deactivator consists in principle of an oscillator, which generates a carrier wave of the desired frequency, and a power amplifier which is so dimensioned that enough power is generated to enable deactivation of even the most insensitive label types, i.e. those with the highest breakdown voltage, at a sufficiently great distance. Although this operative principle is technically satisfactory, the complex composition of this deactivator can sometimes be objectionable. Particularly in applications where adhesive labels of good deactivation sensitivity are used and deactivation from great distances is not required, there is a need for a more economical solution. This is particularly relevant if a deactivation function is to be added to existing packaging table detection devices.